RustyBolt.Info/wordpress

I did a search of my blog and I couldn’t find this topic, so I think that I blogged this ‘way back in my late, great watsonseblog.  So I’ll go over this again.

Sometimes it’s necessary to use a Zener diode for overvoltage protection.  It can also be used as a shunt regulator, but nowadays that’s very inefficient and violates the principle of “green and eco friendly”, so I’ll stick to overvoltage protection.

When a power transistor is driving a variable load with an inductive component, the collector voltage can rise to excessive voltage and damage the transistor.  In order to protect the transistor, the designer can connect a zener diode from collector to emitter.  When the voltage gets as high as the zener’s breakdown voltage, the zener conducts and dissipates the excessive voltage.  This works fine for the typical zener diodes, which come in half watt and 1 watt sizes.  But what happens if the zener has to dissipate more than a half watt or 1 watt?  One way is to put two or more zeners in series.  For instance, a 2N3055 power transistor can handle several amps, and is rated at 60 volts collector to emitter.  I could put six 9V, 1 watt zener diodes in series, and across the emitter to collector.  The six zeners could handle up to 6 watts.  If more power is needed I could connect nine 6V zeners for a total of 9 watts, and other combinations of other voltage zeners could have higher dissipation.

But why do we need to use high powered zeners, when we have a more  than 100 watt transistor right there??  Instead of connecting the zeners collector to emitter, we connect them collector to base.  When the collector voltage rises, the zener starts to conduct, and a small amount of current through the zener to the base turns on the  transistor, where a much larger current goes through the collector to emitter.  In effect, the transistor amplifies the power of the zener.

As a example, we connect a 9V, 1 watt zener from the collector to the base of a 2N3055.  We connect a 1k resistor from the base to the emitter, to prevent any leakage current through the zener from turning on the 2N3055 before the voltage gets to 9V.  As the collector voltage reaches 9V, the zener conducts, and up to 100 milliamps through the zener  is amplified to 3 or more amps through the 2N3055.  That’s up to 30 watts of power, and the 2N3055 must be on a heatsink to keep it cool.

This should work with a transistor that is being driven by a driver circuit.  But when the zener conducts, it is overriding the driver, so the design will have to take that into account.  Also there may be a problem with the high voltage and high power in the transistor, which has a “SOA” or safe operating area, that must not be exceeded.  Exceeding this typically results in a transistor that is permanently damaged.

An alternative is to use a second power transistor with the zener between its collector and base, and 1k resistor between its base and emitter.  Then its emitter and collector are connected to the emitter and collector of the first transistor.  When the voltage rises, the zener conducts and most of the power is dissipated in the second transistor.  I have heard this called an amplified zener, and some other similar names.  But it’s a way to save money on expensive high power zener diodes.